Origin of omphacitites from the Nishisonogi metamorphic rocks, western Kyushu, Japan: comparison with jadeitites

Abstract

Two types of omphacitites are distinguished in ultramafic mélanges that are intercalated with epidote-blueschist facies schists of the Nishisonogi metamorphic rocks in western Kyushu, Japan. One is an omphacitite layer in a metabasite block, and the other is an omphacitite lens in an albitite block. These omphacitites are principally composed of omphacite with minor amounts of winchite/magnesiohornblende, epidote, albite and titanite. Although the petrographic features of both types are similar, the omphacitite layer is depleted in light rare earth elements (LREE; La, Ce and Nd) compared with the omphacitite lens. In the case of jadeitites, those interpreted as being formed by direct precipitation from aqueous fluids are LREE-depleted, while those interpreted as being formed by metasomatic replacement from protoliths are LREE-enriched. Thus, by analogy, the omphacitite layer may have directly precipitated from aqueous fluids, and the omphacitite lens may be metasomatic in origin. The omphacitite layer has amphibole-rich selvages, which are likely formed by the reaction between the omphacitite-forming fluid and the host metabasite. Mass-balance analysis using the isocon method indicates that the formation of the selvages involves additions of large ion lithophile elements (LILE; Rb, K and Ba), Na and Si to the host metabasite. These components were likely introduced via fluid infiltration. Although the source of the omphacitite-forming fluid is not clear, the LILE-rich composition suggests participation of fluids derived from metapelitic rocks.

Fluid-metapelite interaction in an ultramafic mélange: implications for mass transfer along the slab-mantle interface in subduction zones

“…The ultramafic mélanges contain tectonic blocks of metagabbro, mafic schist, pelitic schist, and albitite in a matrix of chlorite-actinolite schist, talc schist, and schistose serpentinite (Nishiyama 1989(Nishiyama , 1990. Small blocks (<5 m in diameter) of jadeitite, omphacitite, rodingite, and zoisitite are found locally (Nishiyama 1978;Shigeno et al 2005Shigeno et al , 2012aMori et al 2011).…”

Jadeitites and associated metasomatic rocks from serpentinite mélanges in the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan: a review

“…The amount of omphacite varies from about 10 to 90 vol% in the omphacite-bearing rocks with the assemblage of omphacite + albite + epidote + amphibole. The amphibole is winchite in the Nishikashiyama omphacitite, and magnesio-hornblende in the Mie omphacitite (Shigeno et al, 2012b). No jadeite is found in these omphacite-bearing rocks, whereas no amphibole occurs in the jadeitite.…”

“…2b of Shigeno et al, 2012b) and metabasite ( Fig. 2a of Shigeno et al, 2012b), or as metric blocks embedded in an actinolite schist matrix (Fig. 5D).…”

“…Mie jadeitite (Nishiyama, 1978;Shigeno et al, 2012b) One of the Mie jadeitite shows a remarkable zoning (Fig. 6D), that consists of jadeitite core (JX) with asymmetric zoning.…”

“…This paper reviews the nature of the Nishisonogi jadeitite in terms of occurrence, petrography, P-T conditions, and geochronology to coincide with the occasion of the selection of jadeitite as the 'national rock' of Japan. The Nishisonogi jadeitite is associated with albitite and omphacite-bearing rocks such as omphacite-bearing metagabbros and omphacitite (Shigeno et al, 2012b), and petrological characteristics of these metasomatic rocks are also briefly discussed. The mineral abbreviations in this paper follows those of Whitney and Evans (2010).…”